The present invention relates to devitrifying-glass formulations and, more particularly, to improved thick film copper via fill inks comprised of devitrifying-glass compositions for use on multilayer alumina-substrate circuit boards.
Thick film copper conductor printing inks comprised of devitrifying glass for use in the fabrication of printed circuit boards are known in the art. The fabricated circuit board is comprised of a dielectric substrate on which is printed one or more conductive metal (e.g., copper) circuits. The use of a devitrifying glass in the thick film copper ink formulation is desirable because its high softening temperature, i.e. about 900.degree. C. or higher, withstands repeated firings at lower temperatures and the conducting metal patterns do not become distorted or diffuse into adjacent layers. Multilayer circuit boards have alternating dielectric/conductor layers.
As is known, a thick film copper ink is comprised of copper powder, a devitrifying glass and a suitable organic medimm.
In the case of multilayer circuit boards, it is necessary to provide via holes filled with copper to electrically interconnect the respective layers to one another. Copper exhibits a relatively high temperature coefficient of linear expansion (i.e., 170-190.times.10.sup.-7 /.degree. C.). However, dielectric material, which is normally employed as a substrate of a printed circuit board, exhibits a much lower temperature coefficient of linear expansion than does copper. For instance, in the case of alumina, which is widely used as a printed-circuit substrate material, the temperature coefficient of linear expansion is about 63.times.10.sup.-7 /.degree. C. at room temperature and rises to about 8.times.10.sup.-7 /.degree. C. at 900.degree. C.
Devitrifying glass, as known in the art, exhibits a relatively low positive temperature coefficient of linear expansion, which is substantially below that of alumina. Specifically, devitrifying glass compositions known in the art exhibit a temperature coefficient of linear expansion in a range of the mid-twenties to the mid-thirties.times.10.sup.-7 /.degree. C. at room temperature, which then rises to about 40.times.10.sup.-7 /.degree. C. at 900.degree. C.
Because the thick-film copper inks known in the past included both devitrifying glass and copper, the printed conductors of the fabricated circuit board exhibited a temperature coefficient of linear expansion significantly below that of copper alone, but still significantly higher than that of an alumina substrate. The difference between the respective temperature coefficients of linear expansion of the alumina substrate and the printed conductor, fabricated from a thick-film printing ink comprises of a prior-art devitrifying glass composition, is most pronounced at higher temperatures of the printed circuit board (i.e., in the range of 600.degree.-900.degree. C.).
In the case of thick-film copper ink via fills for multilayer alumina-substrate circuit boards, the difference in the respective temperature coefficients of linear expansion of the substrate material and the copper (particularly at high temperatures) creates a problem. One adverse effect is the tendency of the copper via fill film made from the ink to blister during fabrication of the multilayer printed circuit board when the temperature is raised to about 900.degree. C. Second, because the mismatch between the two materials, the greater positive temperature coefficient of linear expansion of the copper ink via fill with respect to that of the multilayer alumina substrate tends to cause cracking of the multilayer alumina substrate during fabrication, or thereafter during use, of the multilayer printed-circuit board. This cracking is due to pressure of the copper ink via fill against the walls of the via hole in the alumina substrate.
Even if one could formulate a thick-film copper via fill ink that had a thermal coefficient of linear expansion that closely matched that of alumina at the upper portion of the temperature range between room temperature to 900.degree. C., thereby avoiding the problems of blistering and cracking, a new problem would result at lower temperatures (e.g., room temperature). The new problem is that at such low temperatures, the copper via fill ink then would tend to shrink in the via hole in the alumina substrate, thereby tending to break away from the substrate and fall out of the multilayer printed-circuit board.
What is desired is a thick-film copper ink via fill that has a thermal coefficient of linear expansion that closely matches that of an alumina substrate over the entire temperature range of room temperature to 900.degree. C. The present invention is directed to improved devitrifying glass compositions suitable for use in thick film copper via fill inks, which exhibit a temperature coefficient of expansion that closely matches that of alumina over the entire range of temperatures from room temperature to 900.degree. C.